Rough running, misfire, idle stumble cured with plugs!

On a 94 wagon the 1.9 would run great on cold start but after a brief warmup (maybe a minute) it would develop a misfire. With light throttle it would speed up and become very smooth, again running on all four. But anytime the gas pedal is pushed say 1/8 or 1/4 way down, the engine would run rough again. There was no check engine light, no codes. By pulling plug wires I found that 4 was the offending cylinder. I took the plugs out - but laying them out on the valve cover, they all had big beautiful blue spark - no indication of any spark problem - plugs were clean, gaps were right. Figuring it was a fuel issue instead, like a lazy injector, I pulled out the no.4 injector and it looked drier than the others. But a new injector didn't help. Same problem persisted. After spending the afternoon testing and cleaning the EGR, IACV, MAS, ATS, chasing every possible source of vacuum leaks and re-sealing the injectors, PCV, power brake booster, checking fuel pressure, etc. etc. I was ready to give up. I knew the plugs were fine - but I did another search HERE and found these same symptoms had been cured by new plugs. So I put new plugs in - and IT WAS CURED. Apparently spark gap resistance is lower in air than it is inside the cylinder with air/fuel under pressure. Inside the engine, the spark can take a different path to ground if one exists (like a carbon track through a cracked insulator inside the plug) Next time I'll read here more before diving deep - great forum! Hope this post saves somebody else from a dive.